Elementary steps of NO_x adsorption and surface reaction on a commercial storage-reduction catalyst

The surface species formed during adsorption of NO_x on a commercial NSR catalyst (containing barium oxide, Pt, and alumina as the main components) were investigated by in situ IR spectroscopy. During adsorption of NO, mainly linear and bridged bonded nitrites of Ba-O-N-O-Ba type were formed on Al and Ba oxide components. Nitrites were detected during the initial phase of the NO/O₂ and NO₂ adsorption, whereas with further exposure nitrates were the dominant surface species. Using the different surface species and reaction intermediates identified by IR spectroscopy a series of sequential reaction steps during the sorption of NO_x on a NSR catalyst was derived. Initially, NO is stored in the form of nitrites on the storage component (Ba oxide). NO₂ formed by oxidation on the noble metal component (Pt) sorbs either molecularly by forming nitrate species or dissociatively by forming nitrites. After a certain concentration of NO _x is adsorbed, the transformation and further oxidation of the surface nitrites into surface nitrates by NO₂ occur. The stability of the NO_x surface species was found to increase in the order Al nitrites < Ba nitrites < Al nitrates < Ba nitrates.